1. Field of the Invention
This invention relates to a power amplifier apparatus for used with a 4-channel stereophonic system loaded in a motor vehicle and so on.
2. Description of the Related Art
Known power amplifiers include that disclosed in Japanese Patent Laid-Open Publication No. Hei (JP-A) 6-338738. That power amplifier is adapted for high efficiency operation by selecting a DC output voltage that is very close to the ground potential and driving the load by means of a BTL (balanced transformerless) drive technique, using half-wave signals. FIG. 1 of the accompanying drawings shows a circuit diagram of the known power amplifier.
Referring to FIG. 1, input signal IN is applied to the negative input terminal of a first differential amplifier 1, which produces corresponding opposite phase output signals from its positive and negative output terminals. The positive and negative output signals of the first differential amplifier 1 are amplified by first and second output amplifiers 2 and 3. Said first and second output amplifiers 2 and 3 constitute a BTL amplifier and the load 4 (e.g., a speaker) of the power amplifier is BTL-driven by output signals X and Y of the first and second output amplifiers 2 and 3.
The output signals X and Y of the first and second output amplifiers 1 and 2 are non-linearly added to each other by means of a non-linear adder 5. The non-linear adder 5 is activated to operate as an adder when the output signals of the first and second output amplifiers 2 and 3 are below a predetermined level, and operate as a clamp circuit when the output signals exceed the predetermined level. The output signal of the non-linear adder 5 is applied to the negative input terminal of a second differential amplifier 6, which produces an output signal as a function of the output signal of the non-linear adder 5 and the reference voltage Vref of its positive input terminal. The output signal is then applied to common terminal C of the first differential amplifier 1. The common terminal C is used to determine the output DC voltage of the first and second output amplifiers 2 and 3, in which output DC voltage is controlled as a function of the output signals a and b of the first and second output amplifiers 2 and 3. Therefore, the output DC voltage of the first and second output amplifiers 2 and 3 is selected to be close to the ground potential level and the output signals X and Y of the first and second output amplifiers 2 and 3 are half-wave output signals.
On the other hand, the output signals of the first and second output amplifiers 2 and 3 are added to each other by means of an adder 7, which selects the signal with the higher level from the output signals of the first and second output amplifiers 2 and 3. Switching power source 8 is driven for switching operation according to the output signal z of the adder 7 in order to produce power source voltage Vs for the first and second output amplifiers 2 and 3.
There is also known a 4-channel stereophonic system loaded in a motor vehicle and comprising first and second power amplifiers for amplifying left stereophonic signals and third and fourth power amplifiers for amplifying right stereophonic signals, wherein said first and third power amplifiers are paired to drive the front speakers in the cabin of a motor vehicle while said second and fourth power amplifiers are paired to drive the rear speakers in the cabin of the motor vehicle.
When power amplifies having a circuit configuration as shown in FIG. 1 are used for such a car stereophonic system, a total of four power amplifiers must be installed. If four power amplifiers with the circuit configuration of FIG. 1 are simply combined, a total of four adders 7 must then be used, which makes the entire system bulky and clumsy. Particularly, if the four power amplifiers are integrally arranged on a single substrate, the chip will have a large surface area mainly due to the existence of the adders.